This invention relates to a method for recovering polymers from solution. More particularly, it relates to a method for isolating polymer resin from organic solvents by precipitating the polymer within an aqueous/organic anti-solvent medium.
There are several methods which have been suggested for separating polymers resins from organic solvents, such as those described in U.S. Pat. Nos. 3,590,026; 4,452,976; 3,505,273; 3,954,713; 4,408,040; 4,423,207; 4,205,162 and 2,989,503. In these processes the polymer is isolated by evaporating the organic solvents and precipitating the polymer within an aqueous medium. When utilizing aqueous media to isolate polymer resin, the polymer often forms a large sticky mass prior to precipitation. When formed, there must be sufficient agitation during the process to break up this sticky mass into a particulate product, as noted in U.S. Pat. No. 3,505,273. Breaking up this sticky mass requires a high level of shearing agitation, which is difficult to obtain in large scale operations.
Methods have been disclosed which avoid the formation of this large sticky mass when isolating polymers within aqueous media. These processes often depend on unique process equipment. The processes described in U.S. Pat. Nos. 4,408,040 and 4,423,207 utilize a spray nozzle designed to atomize the polymer solution within an aqueous medium. In the process described in U.S. Pat. No. 4,452,976, a wet pulverizer is used to break up large particles which form within the aqueous medium. Wood discloses the use of a "shearing device" in U.S. Pat. No. 3,267,074 to provide high levels of shearing agitation which prevent the formation of the sticky mass.
Herscovici, U.S. Pat. No. 4,205,162, utilizes a large quantity of water and adds the polymer solution at a slow rate, i.e., dropwise. Although non-shearing agitation is used, quantities of water in the range of 1000 to 2500 parts water per part polymer are used. Handling such large quantities of water on a large scale is difficult and expensive.
Modified aqueous media have been utilized in the prior art to prevent the formation of a sticky mass. Jibben, U.S. Pat. No. 2,989,503, discloses the addition of dimethylbenzene, a non-solvent for polycarbonate, to aid in the production of powdered and granular polycarbonate within aqueous media. The process of Jibben requires a significant expenditure of energy since this non-solvent must be evaporated. In addition, large quantities of a non-solvent are typically utilized. The weight ratio of non-solvent to polymer is greater than 10:1 in the examples provided and the preferred ratio is in the range of 10:1 to 20:1. In U.S. Pat. No. 3,590,026, Carlson et al. disclose the addition of a dispersant to the aqueous medium to prevent agglomeration of the polymers into a sticky mass. Such dispersants include surfactants, for example, potassium and sodium lignin sulfonates, zinc oxide, and water soluble saponified polymers of acrylic acid esters.
Isolation of polymers within organic anti-solvents for the polymer have been disclosed by Wood, U.S. Pat. No. 3,267,074 and Marshall et al., U.S. Pat. No. 4,182,850. However, large quantities of organic anti-solvent are required in such processes, i.e., at least 18 parts by weight anti-solvent per part polymer.
The use of specialized process equipment, high shear agitation, large quantities of anti-solvent and/or water, and large expenditures of energy is avoided in the process of this invention by utilizing both water and an organic anti-solvent, in combination, as the separation medium.